The invention concerns the end face, non-clearance, rotationally rigid connection between the work spindle of a machine tool and the shaft-like workpiece set up between centers on it for machining, especially for tooth grinding.
An end face torque transmission from the work spindle to the workpiece is required in the case of workpieces where the use of a chuck is impossible because, for example, they must be machined on their periphery over their entire length, or because a clamping medium acting on the periphery would collide with the machining tool. This applies chiefly to lathes, circular grinders, but also to gear tooth grinding machines. In the latter case the reason for the need for an end face work driver can additionally derive from the workpiece not being provided with a point suitable for peripheral clamping.
In the applications of face work drivers dealt with here, in contrast to common external and internal chucks, the functions of the centering and supporting of the workpiece against axial and radial machining forces are completely separated from that of the torque transmission, or absorption of the tangential machining forces. The centering and the axial and radial workpiece accommodation are effected by means of pointed work centers on the work spindle and tailstock barrel, and the center bores in the end faces of the workpiece. The non-clearance connection between the work centers and the workpiece center bores is so designed and axially pre-tensioned that the workpiece is held reliably and with high accuracy against the radial and axial machining forces. The function of the face work driver is to transmit the rotation of the work spindle angularly true to the workpiece via the workpiece end face, such that the rotational connection withstands the machining torque reliably, and the function of the centers is not disturbed, especially with respect to locational accuracy. It is easy to appreciate that the angular accuracy of the drive is of extremely great importance in gear tooth grinding, as the tooth flanks must be ground to micrometric precision.
The well known and for most applications established technique of face driving is based on a ring of axially protruding knives of carbide or hardened steel, which bite axially into the softer end face of the workpiece as the workpiece is clamped, and via the thus formed indentations produce a positive connection between the work spindle and the workpiece. In order that in the case of unevenness and axially untrue running of the workpiece end face all knives embed themselves in the workpiece end face and the workpiece is not subjected to axially eccentric loading, the knives are axially movably located in the work spindle, and bear on springs, on a wobble plate or on the fluid of a transmission system. The axial force exerted on the workpiece is divided between the work center and the ring of knives. In order to assure that the proportion on the work center is adequate to fulfill its function, either the work center or the ring of knives must be spring-loaded.
A very advantageous property of this solution in many applications is the transmission of high torques. On account of the geometry of the knives and the constant axial pre-tensioning, the knives bite deeper into the workpiece end face with increasing torque, whereby the resistance to any slip increases. This process is of course accompanied by a slight rotation of the workpiece relative to the work spindle, which is of no disadvantage in most cases of application. It makes this solution unsuitable for tooth flank grinding, however, because here any rotation between the work spindle and the workpiece in the grinding cycle under load results in an unacceptable geometrical tooth flank error.
Moreover conventional face work drivers operate flawlessly with non-hardened workpiece surfaces. On the other hand the penetration of the knives into a hardened workpiece end face is inadequate, and the knife wear increases to an intolerable extent.
It is therefore an object of the invention to introduce a device for the end face driving of a workpiece, which allows the avoidance of these disadvantages, yet permits the driving torques incurred in tooth flank grinding to be transmitted reliably. The object is attained by way of the features described in the independent claim.